1. Field of the Invention
The present invention relates to an apparatus and a method of regenerating a filter for an internal combustion engine provided to trap particulate (particulate matter) included in exhaust gas discharged from a diesel engine, by removing the particulate trapped by the filter in such a manner that the particulate is heated and burnt using a microwave. More particularly, the present invention relates to an apparatus and a method of regenerating a filter, by which heating means and burning means are controlled on the basis of the amount of particulate in the filter during the heating and burning of the particulate.
2. Description of the Related Art
A diesel engine advantageously has higher combustion efficiency and durability as compared with a gasoline engine. On the other hand, the diesel engine has a drawback in that it discharges a large amount of air pollutants. The exhaust gas discharged from the diesel engine includes nitrogen oxide and particulate which is regarded as a significant cause of respiratory illness. For this reason, the regulations of exhaust gas have been made more tight. Against these rigorous regulations, some countermeasures such as an improved combustion method of, for example, delaying the fuel injection time, and a low-sulfurization of light oil are being laid down. Technically, the reduction of the nitrogen oxide and the reduction of the particulate have a trade-off relationship, so that the countermeasures in which the nitrogen oxide is reduced in the vicinity of the engine and the particulate is processed in the exhaust system is currently considered to be the most favorable. There are mainly three kinds of particulate, i.e., SOF (Soluble Organic Fraction), soot, and sulfur compounds. As methods for removing the particulate in the exhaust system, an oxidation catalyst method for reducing SOF, and a method for trapping the particulate with a filter have been developed. The oxidation catalyst method cannot reduce soot, so that it is preferable to use the method using a filter.
However, if the particulate is trapped to the full capacity of the filter, the filter soon becomes clogged. Accordingly, the trapping capability of the filter is lowered and a smooth flow of the exhaust gas is prevented. As a result, the engine output is significantly reduced, or in the worst case, the engine is stopped. Under these circumstances, technological efforts to regenerate the filter have been made worldwide. In the case where the trapped amount is small and the heating is likely to be insufficient, the heated particulate is not completely burnt. This results in an insufficient regeneration. In the case where the trapped amount is large and the heating is likely to be excessive, the filter may be mechanically damaged by the burning at an abnormally high temperature caused by the excess heating. In view of the above cases, the most important practical problem is to ensure the durability of the trapping capability of the filter.
In order to solve the problem, it is necessary to control the operation of the heating means and the supply of gas for burning the heated particulate, based on the heating and burning conditions of the particulate.
It is known that the particulate is burnt at a temperature of 600.degree. C. or higher. As a medium for generating energy to heat the particulate to such a high temperature, various methods have been proposed. Among the proposed methods, a method using a microwave can advantageously reduce the power consumption because the particulate itself is selectively heated.
An apparatus and a method for regenerating a filter by removing the particulate using the microwave as a heating energy is, for example, disclosed in Japanese Laid-Open Patent Publication No. 61-11416. FIG. 12 shows the apparatus disclosed in the above-identified publication. As is shown in FIG. 12, the apparatus includes an engine 1, an exhaust pipe 2, a filter 3, a microwave heating chamber 4, a magnetron 5 as microwave generating means, a microwave shielding device 6 for limiting the microwave heating space, a microwave transmitting device 7 for transmitting the microwave generated by the magnetron 5 to the microwave heating chamber 4, detecting devices 8 and 9, provided in the microwave transmitting paths, respectively, for detecting the microwave supplied to and reflected from the heating chamber 4, and a control device 10 for controlling the operation of the magnetron 5 based on the signals of the detected microwave supplied to and reflected from the heating chamber 4 and the signal indicative of the engine operation time. The apparatus further includes a driving power supply 11, and a muffler 12.
With the above construction, the particulate included in the exhaust gas discharged from the engine is trapped by the filter 3 when the exhaust gas flows through the filter 3. The total amount of the particulate trapped by the filter 3 increases as time elapses. Accordingly, in this process, the magnetron 5 is allowed to operate at a predetermined period in accordance with the output signal from the control device 10. The microwave generated by the magnetron 5 is supplied toward the heating chamber 4. The microwave first enters the heating chamber 4 and reaches a wall of the heating chamber 4 through the interior of the filter 3. Then, the microwave is reflected by the wall of the heating chamber 4 and returned to the magnetron 5. During the transmission of the microwave, the signals of the entering wave into the heating chamber 4 and the reflected wave from the heating chamber 4 are detected. Based on these signals, the change of the microwave characteristics across the heating chamber 4 is measured as a voltage standing-wave ratio.
When the amount of particulate trapped by the filter 3 becomes too large, a load with respect to the engine is increased. In the worst case, the engine may be stopped. Thus, it is necessary to remove the particulate at an appropriate timing. The control device 10 stores the lower limit value of the voltage standing-wave ratio corresponding to a certain amount of the trapped particulate, i.e., the appropriate timing.
When the voltage standing-wave ratio obtained by the detected signals becomes equal to or lower than the lower limit value thereof stored in the control device 10, the output of the magnetron 5 is increased, so as to dielectric-heat the particulate trapped by the filter 3, and also to burn and remove the particulate using the oxygen carried by the exhaust gas. In addition, when the voltage standing-wave ratio during the regeneration reaches or exceeds a predetermined upper limit value thereof, it is detected that the burning and removal of the particulate trapped by the filter 3 is completed, so that the operation of the magnetron 5 is stopped. It is also disclosed that the voltage standing-wave ratio is corrected based on the filter temperature.
The above-described conventional apparatus discloses a method for determining the time at which the filter is regenerated, or a method for determining the time at which the regeneration is terminated. However, the conventional apparatus involves the following problem in the method for controlling the heating, burning, and removal of the particulate.
The problem resides in that the detection signals for determining the time at which the regeneration is terminated are not reliable. This problem may deteriorate the reliability in determining the time at which the next regeneration is started. This problem results from the flow of the exhaust gas through the filter during the dielectric heating by the microwave. The correction of the voltage standing-wave ratio using the filter temperature implies that the trapped amount during the regeneration is fixed. In a method for burning and removing the fixed amount of particulate, when the filter temperature is low, the particulate is subjected to both the heating by the microwave and the cooling by the exhaust gas at the same time. As a result, the particulate in the vicinity of the exhaust-gas upstream side of the filter is not heated to the burning temperature, so as to be left unremoved. The residual particulate in the upstream part of the filter prevents the reflected wave from increasing to a desired level, so that the voltage standing-wave ratio is not increased to the predetermined upper limit value of the voltage standing-wave ratio. Thus, it is difficult to accurately determine the time at which the microwave heating is stopped. When the filter temperature is high (i.e., when the temperature of the exhaust gas is high, and a large amount of particulate is discharged), the heating in the exhaust-gas upstream part of the filter is facilitated, and the particulate in this part enters into a burning condition. In this case, the extent of the burning only depends on the flow amount of the exhaust gas. Therefore, it is impossible to avoid the occurrence of an abnormally high burning temperature. Also, it is impossible to avoid mechanical damage of the filter.
International Publication No. WO 90/01618 shows that the flow amount of the exhaust gas through the filter is reduced during the dielectric heating of the particulate by the microwave. According to the conventional apparatus disclosed in International Publication No. WO 90/01618, the cooling effect by the exhaust gas is reduced when the particulate is initially heated, so that the heating of the particulate in the upstream part of the filter by microwave energy can be facilitated.
In addition, International Publication No. WO 90/01618 discloses that particulate which is previously heated by using the secondary air is burnt. However, as described above, the microwave radiated toward the heating chamber always passes through the filter and reaches a wall of the heating chamber on the opposite side of the filter. This is because the particulate existing in a limited region of the filter cannot absorb all the radiated microwave. Accordingly, the heating chamber may include various regions depending on the electromagnetic field characteristics, such as a particulate trapping region where the temperature rises remarkably, a particulate trapping region where the temperature rises gently, and a region where there is almost no temperature change. It is very difficult to determine the temperature of the particulate which exists in the region where the temperature rises remarkably, especially, the temperature of the particulate which exists in the upstream part of the filter, based on the above-mentioned voltage standing-wave ratio or a reflection coefficient which is extracted from the electromagnetic characteristics across the heating chamber.
Therefore, it is difficult to optimally control the time period for initially heating the particulate, and it is difficult to ensure that the occurrence of any abnormally high burning temperature caused by the supply of exhaust gas or secondary air is avoided, so as to effect stable burning.
In a case where the particulate is burnt by supplying secondary air, the control related to the termination of the secondary air supply is important. However, in the prior art technique, the control is not reliable. Accordingly, there may be a possibility that the exhaust gas of a low temperature is caused to flow through the filter which is in a high temperature condition by the burning particulate. This may result in mechanical damage of the filter due to the thermal stress caused in the filter by the flow of the exhaust gas of the low temperature.
U.S. Pat. No. 5,195,317 proposes an apparatus for detecting the change of electromagnetic distribution in the heating chamber caused by the change in the effective dielectric constant and dielectric loss of the filter along with the increase in the amount of trapped particulate.
However, the detecting section of the apparatus is constructed in such a manner that a slit is provided on the wall of the heating chamber, and the microwave passing through the slit is detected by an antenna. Thus, the construction of the detecting section includes the formation of the slit. This means that the construction of the detecting section is somewhat complicated and large-sized. Moreover, when a plurality of detecting sections are disposed, there arises a problem in that they cannot be arranged close to each other.